Bridge legs in the art for high current low voltage switching contain switches that normally are embodied in the form of a number of parallel metal-oxide-semiconductor field-effect transistors (MOSFETs) for controlling current supplied to an inductive load in the form of a motor. During the last years, on-resistance for the MOSFETs has continually been reduced such that a point has been reached where the switching losses in the motor controller have started to dominate. This makes the benefits of further improvements in MOSFET on-resistance insignificant unless the switching losses can be further reduced.
One factor that makes it difficult to reduce the switching losses is the performance of the MOSFET intrinsic body diode. The performance of this diode has not improved as much as many of the other properties of the MOSFET.
At a sufficient gate voltage, a MOSFET will be turned on with a low on-resistance, for example about 4-5 mohm for a 75 V MOSFET at high junction temperatures. In this on-state, the MOSFET will conduct current both in a forward and a reverse direction. In this context, the forward direction is the “desired” direction, i.e. the direction where the current can be controlled by the switches by applying an appropriate control signal to a respective switch control gate, whereas the reverse direction is the “undesired” direction where the current cannot be controlled by the switches by applying an appropriate control signal to a respective switch control gate.
However, the MOSFET transistor has an intrinsic body diode that conducts current in the MOSFET reverse direction if the transistor is in off state. In order to avoid current shoot through in a bridge leg, a deadband in time is used between control signals for switching the respective MOSFET, typically around 1-2 us. During this interval the current will be transferred from the channel in the MOSFET to the intrinsic body diode of either of the two switches of the bridge leg depending on direction of load (motor) current. After the dead-band time the other switch in the bridge leg is turned-on. Now the reverse recovery current will increase to a value higher than load (motor) current in order to recharge the diode and extinguish the current through the diode.
Hard switching of inductive load, as described above, will generate diode recovery of transistor body diode or separate anti-parallel connected diode. This diode recovery will generate shoot through currents with uncontrolled di/dt during the recovery part of the switching in the bridge leg, which generates EMC emissions. Traditionally, these conventional switches have had a limited upper switching rate of approximately 10-25 kHz and have required a relatively large heat-sink in order to dissipate losses resulting from current conduction and switching.